Fiber optic directional couplers are used to couple optical electromagnetic energy from one fiber optic strand to another. Each strand is typically formed of a core covered by cladding, the index of refraction of the core being greater than that of the cladding so that the energy is guided within the strand with minimum loss.
In single mode fibers the core and cladding indices of refraction and the core radius are selected so that the energy is contained primarily within the core. In contrast to multimode fibers, in which the core size is typically about 50 to 100 microns in diameter, in single mode fiber the core diameter is typically 5 to 10 microns in diameter.
Various kinds of couplers have been designed in the past to couple electromagnetic energy from one strand to another. Some couplers fuse the fibers together, for example, see Canadian Pat. No. 1,119,860 issued Mar. 16, 1982 entitled LOW LOSS ACCESS COUPLER FOR MULTIMODE OPTICAL FIBER DISTRIBUTION SYSTEMS by Kawasaki et al; U.S. Pat. No. 3,931,518 to Miller; U.S. Pat. No. 4,019,015 to Miller; U.S. Pat. No. 4,307,933 to Palmer and 4,336,047 which relate to single or multimode directional couplers.
In U.S. Pat. No. 4,493,528 to Shaw a single mode fiber optic coupler is described of a kind to which the present invention is directed. In that patent a pair of oppositely curved fibers are placed in contact, but prior to contact, adjacent portions of the cladding are ground off and are optically polished. The grinding and polishing are conducted with the fiber strands embedded in slots in glass blocks or substrates with the adjacent surfaces of the glass substrates being ground at the same time as the cladding of the fibers. Once grinding and polishing have been completed, the two substrates are placed together face to face whereby the fibers come into contact, resulting in energy from one fiber being transferred to the other once energy is being carried by an input fiber.
The glass substrate and fiber are ground and polished together until the fiber cores are in close proximity, i.e. within a few microns, of the glass substrate surface. In order to transfer the light energy between the two single mode fibers in the directional coupler, the separation of the fiber cores should of the order of a few fiber core diameters.
The polishing of the glass block is a difficult process. Not only must the flatness of the block be maintained but the depth of the fiber core below the block surface must be closely monitored. The final stage of polishing is a step-by-step process requiring the measurement of the core depth followed by the removal of only a few microns of material at a time.
Clearly the tolerance of polishing is very critical. If one overpolishes, too much coupling will be achieved and the energy will be coupled back into original fiber, if there is too little polishing, there will be no or little coupling achieved. There is a very narrow window of critical depth to which the fibers may be successfully ground and polished, which is typically a few microns or less.
The polishing process is further complicated in the case of directional couplers made from polarization maintaining fiber. Such single mode fibers often contain stress members, which are larger than the fiber core, and are located just outside but close to the fiber core. During the polishing process removal of part of the stress member can often result in fracture of the fiber core.
The process of polishing to a precise depth is very labor intensive and results in a low yield of useful couplers due to fiber fracture during polishing or polishing errors which results in the removal of too much material.